The invention is based on a method for transformation encoding of moving-image sequences, in the case of which motion vectors are estimated block-by-block between a reference image signal and an actual image signal from the image sequence, with which said motion vectors the motion compensation is carried out.
In the case of hybrid encoding concepts for encoding moving-image sequences, a motion vector field is estimated block-by-block between a previously-generated image signal (reference frame) and an actual frame from an image sequence. This vector field is then used to perform a motion compensation. The motion vector field and the residual prediction error are encoded and transmitted to the receiver. The prediction error is usually encoded using block transformations, typically a discrete cosine transformation (DCT) with 8×8 coefficients.
An 8×8 DCT is used for transformation encoding in the previously standardized methods for moving-image encoding [1, 2]. For the motion compensation, blocks that are 16×16 and 8×8 pixels in size are used and, with MPEG-4, 16×8 pixels [1] are also used in the case of interlaced coding. The size of the block transformation is constant with 8×8 coefficients.
In the test model for the new H.26L video coding standard [3], a 4×4 integer transformation based on the DCT is proposed. Compared to the DCT, this has the advantage that the pixel values—present in the form of integers—are mapped on integer transformation coefficients. This makes perfect reconstruction possible on the one hand and, on the other, it eliminates the transformation errors that are possible with the heretofore-common float-DCT, which said errors occur when the inverse DCTs are implemented differently in the transmitter and the receiver, e.g., when the float data type is used on one side and the double data type is used on the other side. An integer transformation is presented in [4] that approximates the transformation properties of the DCT and can be used in place of the float DCT.
In the H.26L test model, block sizes of 16×16 to 4×4 pixels are used for motion compensation. They are divided into 4×4 blocks in the test model for transformation encoding.